Initial growth effects on the properties of GaN buffer layer and subsequent GaN overlayer by MOCVD

The role of temperature ramping rate during the two-step growth of GaN-on-sapphire by metalorganic chemical vapor deposition is explored. The surface morphology and crystalline properties of the GaN buffer layer annealed under various temperature ramping rates (20-60 °C/min) were investigated by atomic force microscopy and x-ray measurements. For the lower ramping rates employed, a dramatic re-evaporation of the GaN buffer layer was observed. This makes the buffer layer thinner, yielding the GaN epilayer of hexagonal morphology. However, as the higher ramping rates applied, the surface becomes rougher and exhibits hexagonal three-dimensional islands. It could be due to the fact that the grains of the GaN buffer layer have no enough time to coarse. Under a temperature ramping rate of 40 °C/min, a smooth buffer-layer surface can be maintained and result in a subsequent high-quality over-layer deposition. The mirror GaN epilayer shows a near-band-edge peak (25 K) centered at 3.477 eV with a full width at half maximum as narrow as 13.1 meV. The observed temperature-ramping-rate effects can be interpreted by the coalescence mechanism of the GaN buffer layer involving Ostwald ripening, sintering and cluster migration.